Solids, Liquids and Gases

Everything is made of something

Look around you. A desk. A cup of water. The air you breathe.

All of these are matter. Matter is anything that takes up space and has weight. Your pencil is matter. The sea is matter. Even the air in a balloon is matter, although you cannot see it.

Scientists sort matter into three groups called states:

• Solids, like stones, wood and ice.
• Liquids, like water, palm oil and milk.
• Gases, like the air around you and the steam from a pot.

In this book you will learn how to tell the three states apart, and how heating and cooling can change one into another.

Try this: Name one solid, one liquid and one gas you can find in your kitchen.

Solids

A solid keeps its own shape.

Put a stone on the table. It stays a stone. Put it in a cup. It is still the same shape. You can pick it up and hold it.

Solids:

• have a fixed shape;
• take up a fixed amount of space (a fixed volume);
• can be hard (a rock) or soft (a sponge), but they still keep their shape.

Examples: a wooden stool, a coin, a book, a piece of chalk, a kenkey ball, a brick, your bones.

Some solids can be bent or squashed, like rubber or cloth, but they do not flow and they do not spread out to fill a container.

Try this: Find five solids in your classroom. Say what each one is made of.

Liquids

A liquid flows and takes the shape of its container.

Pour water into a cup: it is cup-shaped. Pour the same water into a bowl: now it is bowl-shaped. The amount of water stays the same, but its shape changes.

Liquids:

• have no fixed shape;
• take up a fixed amount of space (fixed volume);
• can be poured;
• are wet.

Examples: water, palm oil, milk, petrol, juice, honey, groundnut soup.

Some liquids flow quickly, like water. Some flow slowly, like honey. Both are liquids.

Try this: Pour some water from a cup into a plate. What shape is it now? Pour it back. Did any of the water change?

Gases

A gas spreads out to fill all the space it can.

Blow up a balloon. The air you blow in spreads out to fill the whole balloon. Let it go, and the air rushes out and spreads into the room.

Gases:

• have no fixed shape;
• have no fixed volume: they spread to fill any container;
• are usually invisible;
• can be squashed into a smaller space.

Examples: the air around you, the steam above a boiling pot, the gas in a cooking cylinder, the bubbles in a fizzy drink.

Air is a mix of gases. You cannot see it, but you can feel it when the wind blows, and you can see it move a flag.

Try this: Wave a piece of paper near your face. What do you feel?

Three states, side by side

	Solid	Liquid	Gas
Keeps its shape?	Yes	No	No
Can be poured?	No	Yes	No (it spreads)
Fills its container?	No	Fills the bottom	Fills all of it
Can you see it?	Yes	Yes	Usually no
Example	stone	water	air

Here is a quick test:

1. Does it keep its shape on its own? → Solid.
2. Can you pour it, and does it sit in the bottom of a container? → Liquid.
3. Does it spread out everywhere and is hard to see? → Gas.

Try this: Use the test on these: sand, milk, smoke, a spoon, honey, the air in a tyre.

Water in three states

Water is special. We see it in all three states every day.

• Ice is water as a solid. It keeps its shape. It is cold and hard.
• Water is the liquid. It flows. We drink it.
• Steam (water vapour) is water as a gas. It rises from a boiling pot and spreads into the air.

It is all the same stuff. Only the state is different.

Ama puts a sachet of water in the freezer. By morning it is a solid block. She leaves it in the sun. By afternoon it is liquid again. Her mother boils some for tea, and steam rises up.

Try this: Draw three pictures: ice, water, steam. Label each one with its state.

Melting

Melting is when a solid turns into a liquid. It happens when the solid gets warm enough.

Put an ice cube on a plate in the sun. Slowly it gets smaller and a puddle grows around it. The ice is melting. Solid → liquid.

Other things melt too:

• Butter melts in a hot pan.
• Candle wax melts near the flame.
• Chocolate melts in your hand.
• Shea butter melts on warm skin.

Each solid has its own melting temperature. Ice melts at 0 °C. Butter needs a little more heat. Iron needs a very, very hot fire.

Try this: Hold a small piece of ice in your hand for one minute. What happens? Where did the heat come from?

Freezing

Freezing is the opposite of melting. A liquid turns into a solid when it gets cold enough.

Kofi fills an ice tray with water and puts it in the freezer. The freezer takes heat away from the water. A few hours later the water is hard ice. Liquid → solid.

Water freezes at 0 °C. That is why we call 0 °C the freezing point of water.

Other liquids freeze too, but at different temperatures. Palm oil goes solid and cloudy on a cool morning. Melted candle wax goes hard again as it cools.

Freezing does not need a freezer. It just needs the liquid to lose enough heat.

Try this: Watch a bottle of palm oil on a cool morning and on a hot afternoon. Describe the difference.

Melting and freezing go both ways

Melting and freezing are a pair. Heat goes in: solid melts to liquid. Heat goes out: liquid freezes to solid.

Ice → (warm) → water → (cold) → ice → (warm) → water …

You can go round and round as many times as you like. The water is not used up. It only changes state.

Abena's ice-cream melts in the sun. If she puts the melted ice-cream back in the freezer, it goes hard again. It will not look as nice, but it is the same stuff.

This is a reversible change. It can be undone.

Try this: Melt a little butter in a spoon over a candle (ask an adult). Let it cool. What happens?

Boiling and evaporating

A liquid can turn into a gas. This is called evaporation.

When water gets hot enough, it boils. Bubbles rise and burst, and steam rushes out. Water boils at 100 °C. Liquid → gas.

But water does not have to boil to become a gas. Wet clothes on the line dry even on a cool day. The water slowly turns into vapour and drifts away. That is evaporation too, just slower.

Puddles dry up after rain. A wet blackboard dries. Water in an open cup slowly goes down. All evaporation.

Try this: Put a little water in a saucer and leave it in the sun. Check it every hour. Where did the water go?

Condensing

A gas can turn back into a liquid. This is called condensation. It happens when the gas gets cooler.

Hold a cold metal spoon above a boiling pot (carefully). Drops of water appear on the spoon. The steam touched the cold spoon, lost heat and turned back into liquid water. Gas → liquid.

You see condensation all the time:

• Drops on the outside of a cold bottle of water. Water vapour in the air cooled on the bottle.
• A foggy mirror after a hot bath.
• Morning dew on grass.
• Clouds! They are tiny drops of water that condensed high in the cold sky.

Try this: Breathe on a cold window or mirror. What appears? Where did it come from?

The water cycle

Evaporation and condensation drive the water cycle.

1. The sun heats the sea, rivers and Lake Volta. Water evaporates into the air as vapour.
2. The vapour rises. High up it is cold, so it condenses into tiny drops. These make clouds.
3. The drops join together and get heavy. They fall as rain.
4. Rain fills the rivers and the lake. The sun shines, and it starts again.

The same water has been going round for millions of years. The rain that falls on your school may once have been in the sea at Elmina.

Try this: Draw the water cycle as a circle with four pictures: sun on the sea, cloud, rain, river.

Heat makes the change

Here is the big idea of this book. Heat is what changes the state of matter.

Add heat: solid → liquid → gas (melt) (evaporate)

Take heat away: gas → liquid → solid (condense) (freeze)

Ice in a pot on the fire: first it melts to water, then the water boils to steam. Steam on a cold lid: it condenses to drops, and in a freezer the drops would freeze to ice.

Nothing is created or destroyed. The water only gains or loses heat.

Try this: Copy the two arrows above into your book. Under each arrow, write one example from your own home.

Cooking is changing states

Kofi's mother is cooking. Watch the states change.

• She puts solid shea butter in the pan. It melts into a liquid.
• She pours in liquid water. It boils and steam rises as a gas.
• She drops in solid rice. The rice stays solid but soaks up water.
• The steam touches the cold lid and turns back to drops of liquid water. They drip back into the pot.

Candle wax, butter, chocolate and ice all melt at low heat. Metal pots do not melt on a cooking fire; they need much more heat. That is why they are good for cooking!

Try this: Next time someone cooks, list every solid, liquid and gas you notice.

Candle wax: solid, liquid, gas

A burning candle shows all three states at once.

• The candle itself is solid wax.
• Near the flame, the heat melts the wax into a small pool of liquid.
• The liquid wax travels up the wick, gets very hot and turns into a gas. The gas is what actually burns.
• Drips that run down the side cool and freeze back into solid wax.

When you blow out the candle, you see a thin trail of smoke. That is wax vapour cooling in the air.

Ask an adult before you look closely at a candle. Never touch the flame or the melted wax.

Try this: Watch a candle for two minutes with an adult. Point to the solid wax, the liquid wax and where the gas burns.

Sachet water: a cold experiment

Pure water sachets are everywhere in Ghana. They are perfect for an experiment.

1. Feel a sachet from the shop. The water inside is a liquid. It flows when you squeeze it.
2. Put it in a freezer overnight. In the morning it is a hard block: a solid. Notice it got slightly bigger. Water swells when it freezes!
3. Leave it on a plate in the shade. Watch drops form on the outside. That is condensation from the air.
4. After a few hours the block has melted. It is a liquid again.

You saw freezing, condensation and melting with one sachet of water.

Try this: Time how long the frozen sachet takes to melt in the shade. Then try one in the sun. Which is faster, and why?

Is it a solid? Tricky cases

Some things are hard to sort.

Sand. You can pour sand, so is it a liquid? No. Look closely: each grain is a tiny solid that keeps its shape. Sand is many small solids.

Powder, flour, sugar. The same. Tiny solids.

Sponge or foam. Soft, but keeps its shape. A solid with air holes.

Jelly. Wobbly, but keeps its shape on a plate. A soft solid.

Toothpaste. Flows slowly out of the tube, then holds its shape on the brush. Scientists call this a very thick liquid.

Rule of thumb: look at the smallest piece. If each tiny piece keeps its shape, it is a solid.

Try this: Pour some sugar, then some water. Describe how each one moves.

Is it a liquid? Thick and thin

Not all liquids flow the same way.

Water pours fast. Honey pours slowly. Palm oil is in between. Engine oil is slow and sticky.

How easily a liquid flows is called its viscosity. Thick liquids like honey have high viscosity. Thin liquids like water have low viscosity.

Heat changes viscosity too. Cold honey hardly moves. Warm honey runs easily. That is why shea butter is hard in the morning and soft by afternoon.

Still, all of these are liquids: they take the shape of the container and sit in the bottom.

Try this: Tilt a spoon of water, a spoon of honey and a spoon of palm oil. Which reaches the edge first? Put them in order from fastest to slowest.

Is it a gas? Seeing the invisible

Gases are hard to notice because most are invisible. But you can prove they are there.

• Blow up a balloon. Something fills it. That something is gas.
• Turn an empty cup upside down and push it into water. The water cannot get in. The "empty" cup is full of air.
• Open a fizzy drink. The bubbles are gas escaping from the liquid.
• Fan yourself. You feel air moving on your skin.
• Watch smoke from a fire. The smoke is tiny solid bits carried by hot gas, so you can see which way the gas goes.

Gases have weight too. A full football is heavier than a flat one.

Try this: Try the upside-down cup in a bucket of water. Is the inside wet or dry when you lift it out?

Air is all around us

The most important gas is the air we breathe. It is a mix of gases: mostly nitrogen and oxygen.

• Your lungs take oxygen from the air.
• Fire needs oxygen to burn. Cover a flame with a cup and it goes out.
• Air fills tyres, footballs and balloons.
• Wind is moving air. It pushes sails, dries clothes and turns windmills.

Air is everywhere, even in "empty" rooms, in the soil and in water (that is how fish breathe).

Air can be squashed. Press a football pump and you push the air into a smaller space. Liquids and solids cannot be squashed like that.

Try this: Hold your breath for ten seconds. What does your body want? That is how important this gas is.

Steam is not smoke

People often mix these up.

Steam is water as a gas. It comes from boiling water. It is clean, and when it cools it turns back to water drops. You can see a white cloud above a kettle; that cloud is actually tiny drops of condensed water, not the gas itself. The real steam, right at the spout, is invisible.

Smoke comes from burning. It is tiny bits of solid ash and soot floating in hot gas. It is dirty and smells.

Steam from a pot of rice: safe to smell, but very hot. Smoke from a fire: do not breathe it in.

Try this: Look above a boiling pot. Find the clear, invisible gap just above the water before the white cloud starts.

Changes you can undo

Melting, freezing, evaporating and condensing are reversible changes. You can go back.

Ice → water → ice. Water → steam → water.

Nothing new is made. It is still water.

Some changes are not reversible. When you cook an egg, it goes from runny to solid. But you cannot cool it back into a raw egg. When wood burns, it becomes ash and gas. You cannot un-burn it.

Changing state is reversible. Cooking and burning are not.

Try this: Sort these into "can undo" and "cannot undo": melting chocolate, frying plantain, freezing juice, burning paper, boiling water.

Temperature and the thermometer

We measure how hot something is with a thermometer. The unit is degrees Celsius, written °C.

Useful numbers for water:

• 0 °C: water freezes, ice melts.
• 100 °C: water boils.
• About 37 °C: your body.
• About 30 °C: a hot afternoon in Accra.

Ice from the freezer may be below 0 °C. As it warms up it reaches 0 °C and starts to melt. It stays at 0 °C until all the ice has melted, then the water warms further.

Try this: If you have a thermometer, measure a cup of tap water, a cup of iced water and a cup of warm water. Write down each temperature.

Keeping things cold, keeping things hot

If heat changes state, we can slow the change by keeping heat out, or in.

• Cool box. Thick walls stop heat getting in, so the ice inside melts slowly. Sachet sellers use them.
• Flask. Keeps tea hot by stopping heat escaping.
• Shade. The sun's heat cannot reach the ice.
• Cloth wrap. Wrapping a block of ice in cloth slows melting.
• Lid on the pot. Traps steam, so the water does not all evaporate away.

Materials that slow heat are called insulators: foam, cloth, wood, air gaps. Metal lets heat through quickly.

Try this: Put one ice cube in a cup wrapped in cloth and one in a bare cup. Which melts first?

States of matter in the weather

The weather is states of matter at work.

• Rain is liquid water falling from clouds.
• Clouds and fog are tiny drops of liquid floating in the air.
• Dew is water that condensed on cool grass at night.
• Harmattan dust is tiny solids carried by dry air.
• Hail, which is rare in Ghana, is frozen rain: solid.
• Humidity is water vapour, a gas, in the air. On a sticky day there is a lot of it.

In the rainy season the air is full of water vapour. In the Harmattan the air is dry, so wet clothes dry fast and lips crack.

Try this: Each morning for a week, write down whether you see dew, fog, rain or dust.

Solids, liquids and gases at the market

Walk through the market with Abena.

• Solids: yams, tomatoes, cloth, pots, charcoal, bread, dried fish.
• Liquids: palm oil in bottles, groundnut oil, fresh milk, drinks, kerosene.
• Gases: the air, smoke from the roasting plantain, the gas cylinder at the chop bar, the smell of fresh bread (smells travel because they are gases!).

The ice seller keeps ice blocks in a cool box. The sachet water is cold in a bucket. The pot of light soup bubbles and steams.

Try this: Next time you go to the market, write down ten things and sort them into solids, liquids and gases.

Materials and their states

A material is what something is made of. Most materials are solid at room temperature, but each has its own melting point.

Material	State at room temperature	Melts at
Ice	solid (if kept cold)	0 °C
Butter	solid	about 35 °C
Candle wax	solid	about 60 °C
Plastic bag	solid	about 120 °C
Iron	solid	about 1500 °C

That is why a plastic spoon goes soft in a hot pot but an iron spoon does not.

Some materials are liquid at room temperature: water, oil, mercury in old thermometers. Some are gas: air, cooking gas.

Try this: Why should you never leave a plastic plate near the cooking fire?

Experiment: the melting race

You need: three small pieces of ice the same size, three saucers.

1. Put one saucer in the sun, one in the shade, one in a fridge (if you have one).
2. Start a clock.
3. Check every five minutes. Write down which ice has melted most.

What to expect: the ice in the sun melts first, because it gets the most heat. The ice in the fridge melts last, or not at all.

Write it up:

• What I did
• What I saw
• What I learned

This is how real scientists work: a question, a fair test, careful watching, and a conclusion.

Experiment: disappearing water

You need: two cups, water, a plate.

1. Pour the same amount of water into both cups.
2. Cover one cup with the plate. Leave the other open.
3. Put both in the sun for a day.

What to expect: the open cup has less water at the end. The covered cup has about the same, and there may be drops on the underside of the plate.

Why: water in the open cup evaporated into the air. In the covered cup, the vapour hit the plate, condensed and fell back.

Try this: Mark the water level with a pen at the start so you can measure the change.

Melting is not dissolving

People sometimes say sugar "melts" in tea. It does not. It dissolves.

Melting needs heat and changes a solid into a liquid. Ice melts into water. Nothing else is needed.

Dissolving needs a liquid to mix into. Sugar in water breaks into tiny bits too small to see and spreads through the water. The water tastes sweet, but the sugar is still sugar.

Test: a sugar cube on a warm plate stays a cube. A sugar cube in warm water disappears. The first is not melting, because sugar needs a much hotter oven to melt. The second is dissolving.

Salt, sugar and Milo dissolve. Sand and stones do not.

Try this: Stir a spoon of salt into a cup of water. Can you see it? Can you taste it? Where did it go?

Experiment: the balloon and the bottle

Gases spread out when they get warm. Here is a way to see it.

You need: an empty plastic bottle, a balloon, a bowl of hot water (ask an adult), a bowl of cold water.

1. Stretch the balloon over the mouth of the empty bottle. The bottle looks empty, but it is full of air.
2. Stand the bottle in the hot water. Wait. The balloon slowly rises and starts to fill.
3. Move the bottle to the cold water. The balloon sinks again.

Why: when the air inside warms up, it spreads out and needs more room, so it pushes into the balloon. When it cools, it shrinks back.

Nothing was added to the bottle. Only heat went in and out.

Try this: Draw the bottle in hot water and in cold water. Label the air in each one.

Quiz: which state?

Write solid, liquid or gas.

1. A brick
2. Palm oil
3. The air in a tyre
4. A ball of kenkey
5. Steam from a kettle
6. Milk
7. A block of ice
8. The bubbles in a fizzy drink
9. Honey
10. A rubber slipper

Now write melting, freezing, evaporating or condensing.

11. Ice turning to water
12. Puddles drying after rain
13. Drops forming on a cold bottle
14. Water turning to ice in the freezer
15. Butter going soft in a hot pan

Safety with heat

Changing states often needs heat, and heat can hurt.

• Steam burns. It is hotter than boiling water. Never put your hand over a steaming pot.
• Melted wax and hot oil burn. Keep away from the pan.
• Ice can hurt too. Very cold ice sticks to wet skin. Do not hold it for a long time.
• Gas cylinders must be turned off after cooking and kept away from flames.
• Never do heating experiments without an adult.

A good scientist is a careful scientist.

Try this: Make a poster with three safety rules for the kitchen.

What you have learned

You can now:

• Say that all matter is a solid, a liquid or a gas.
• Describe each state: solids keep their shape, liquids flow and take the shape of their container, gases spread out to fill the space.
• Explain melting (solid → liquid), freezing (liquid → solid), evaporating (liquid → gas) and condensing (gas → liquid).
• Say that heat going in or out causes these changes, and that they are reversible.
• Give everyday examples: ice, sachet water, shea butter, candle wax, steam from the pot.
• Describe the water cycle.

Next you will learn about mixing materials and dissolving. The same careful watching will help you there.

Learn more

• DK Find Out: Solids, liquids and gases — clear pictures and short facts for children
• Ducksters: Solids, liquids and gases — a simple reading page with a quiz
• Science Kids — experiments, facts and games for young scientists
• National Geographic Kids — science stories and videos
• NaCCA Ghana — the official Basic 3 science curriculum

Watch more

Think about it

1. Why does a wet school uniform dry faster on a windy day than on a still day?
2. If you leave a frozen sachet of water on a plate, where does the water on the outside of the sachet come from?
3. Sand can be poured. Why do scientists still call it a solid?
4. What would happen to the water cycle if the sun stopped shining?
5. Why is a metal spoon a bad choice for stirring a hot pot if you want to hold it for a long time?